JP2007270325A - Steel plate superior in fine blanking workability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel plate which is superior in FB workability and further in formability after the FB working step, and to provide a manufacturing method. <P>SOLUTION: The steel plate has a composition comprising, by mass%, 0.1-0.5% C, 0.5% or less Si, 0.2-1.5% Mn, and P and S controlled into an appropriate range; and has a structure including ferrite of which the average particle diameter is 10 μm to 20 μm, and carbides in ferrite grains of which the average particle diameter is 0.3 to 1.5 μm. Then, the steel plate acquires superior FB workability and formability (side bending extensibility) after the FB working step, and gives the long life to the die. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel plate suitable for use in automobile parts and the like, and is particularly excellent in fine blanking workability suitable for use in which precision punching (hereinafter also referred to as fine blanking or FB processing) is performed. Related to the steel plate.

When manufacturing complex machine parts, it is known that fine blanking is a very advantageous processing method compared to cutting from the viewpoint of improving dimensional accuracy and shortening the manufacturing process.
In normal punching, the clearance between tools is about 5 to 10% of the thickness of the metal plate that is the material to be punched. Unlike normal punching, fine blanking is the clearance between tools. Is set to an extremely small value (actually about 2% or less of the thickness of the metal plate that is the material to be punched), and punching is performed by applying a compressive stress to the material near the tool cutting edge. And fine blanking is
(1) Suppression of cracks from the tool cutting edge is suppressed, the fracture surface seen in normal punching is almost zero, and a smooth machined surface with a machined surface (punched end surface) of almost 100% shear surface is obtained. ,
(2) Good dimensional accuracy,
(3) It has a feature such that a complicated shape can be overcome in one step. However, in the fine blanking process, the degree of processing that the material (metal plate) receives is extremely severe. Further, in the fine blanking process, since the clearance between tools is almost zero, there is a problem that the load on the mold becomes excessive and the mold life is shortened. For this reason, materials to which fine blanking processing is applied have been required to have excellent fine blanking workability and to prevent a decrease in mold life.

  In response to such a request, for example, Patent Document 1 includes a composition containing C: 0.15 to 0.90 wt%, Si: 0.4 wt% or less, Mn: 0.3 to 1.0 wt%, and a spheroidization rate of 80% or more, A high-carbon steel sheet having a structure in which carbide having an average particle diameter of 0.4 to 1.0 μm is dispersed in a ferrite matrix and having a notch tensile elongation of 20% or more and excellent in precision punching workability has been proposed. According to the technique described in Patent Document 1, the precision punchability is improved, and the die life is also improved. However, the high carbon steel sheet described in Patent Document 1 has a problem that the formability after fine blanking is inferior.

  Patent Document 2 includes C: 0.08 to 0.19%, Si, Mn, and Al in appropriate amounts, Cr: 0.05 to 0.80%, and B: 0.0005 to 0.005%. A steel sheet for precision punching that has been rolled into a steel sheet has been proposed. The steel sheet described in Patent Document 2 has low yield strength, high impact value, excellent fine blanking workability, high low strain area n value, excellent composite forming workability, and quick heat hardenability for a short time. It is said that it is an excellent steel plate. However, Patent Document 2 does not show a specific evaluation for fine blanking workability. Moreover, the steel plate described in Patent Document 2 has a problem that the formability after fine blanking is inferior.

  Patent Document 3 contains C: 0.15 to 0.45%, has a composition in which the contents of Si, Mn, P, S, Al, and N are adjusted to an appropriate range, and further has a pearlite + cementite fraction of 10 %, And a high carbon steel sheet having a structure in which the average grain size of ferrite grains is 10 to 20 μm and excellent in formability in rolling and fine blanking is proposed. The high-carbon steel sheet described in Patent Document 3 is excellent in fine blanking workability and further improves the mold life in fine blanking work. However, the high carbon steel sheet described in Patent Document 3 has a problem that the formability after fine blanking is inferior.

  Furthermore, all of the steel sheets described in Patent Document 1, Patent Document 2, and Patent Document 3 have satisfactory fine blanking workability in fine blanking processing under recent severe processing conditions. However, the mold life has not been improved sufficiently, and the moldability after fine blanking has been poor.

  Initially, fine blanking has been applied to parts that are not processed after fine blanking, such as gear parts. Recently, however, the application of fine blanking to automotive parts (reclining parts, etc.) has been expanding. Considering application to parts that require stretch flange processing or overhanging after fine blanking. Has been. For this reason, as an automobile part, a steel sheet that is excellent in fine blanking workability and excellent in forming workability such as stretch flange processing and overhang processing after fine blanking processing is eagerly desired.

  Many proposals have been made as techniques for improving stretch flangeability. For example, Patent Document 4 includes C: 0.20 to 0.33%, a composition containing Si, Mn, P, S, sol. Al, and N in an appropriate range, and further containing Cr: 0.15 to 0.7%. There has been proposed a hot-rolled steel sheet for wear resistance that has a mixed structure of ferrite and bainite that may contain pearlite and has excellent stretch flangeability. In the hot-rolled steel sheet described in Patent Document 4, the above-described structure increases the hole expansion rate and improves stretch flangeability. Further, Patent Document 5 has a composition containing C: 0.2 to 0.7%, a structure in which the average particle size of carbide is 0.1 μm or more and less than 1.2 μm, and the volume fraction of ferrite grains not containing carbide is 15% or less. There has been proposed a high carbon steel sheet having excellent stretch flangeability. In the high carbon steel sheet described in Patent Document 5, the generation of voids at the end face during punching can be suppressed, the growth of cracks in the hole expanding process can be slowed, and the stretch flangeability is improved.

Patent Document 6 discloses a punching property having a composition containing C: 0.2% or more, having a structure mainly composed of ferrite and carbide, having a carbide particle size of 0.2 μm or less and a ferrite particle size of 0.5 to 1 μm. A high carbon steel plate excellent in hardenability has been proposed. Thereby, both the punchability determined by the burr height and the mold life and the hardenability are improved.
JP 2000-265240 A JP 59-76861 A Japanese Patent Laid-Open No. 2001-140037 Japanese Unexamined Patent Publication No. 9-49065 JP 2001-214234 A Japanese Patent Laid-Open No. 9-316595

  However, both of the techniques described in Patent Document 4 and Patent Document 5 are based on the premise that the conventional punching process is performed, and the application of the fine blanking process in which the clearance is almost zero is not considered. Absent. Therefore, it is difficult to ensure the same stretch flangeability after severe fine blanking, and there is a problem that the mold life is shortened even if it can be ensured.

Moreover, in the technique described in Patent Document 6, it is necessary to make the ferrite grain size in the range of 0.5 to 1 μm, and it is difficult to industrially manufacture a steel plate having such a ferrite grain size. There has been a problem that it leads to a decrease in product yield.
The present invention has been made in view of the above-mentioned problems of the prior art, and provides a steel plate excellent in fine blanking workability and further excellent in forming workability after fine blanking and a method for producing the same. For the purpose.

In order to achieve the above-mentioned object, the present inventors have intensively studied the influence of the metal structure on the fine blanking workability (hereinafter abbreviated as FB workability), particularly the influence of the morphology and distribution state of ferrite and carbide. did.
As a result, it was found that the FB workability, the molding workability after FB processing, and the mold life were closely related to the carbide grain size and ferrite grain size present in the ferrite grains. The steel material having a composition in a predetermined range is subjected to hot rolling finish rolling conditions and subsequent cooling as appropriate conditions to obtain a hot rolled steel sheet having a nearly 100% pearlite structure, and further subjected to appropriate conditions of hot rolled sheet annealing. FB workability by making the metal structure a ferrite + cementite (granular carbide) structure with an average ferrite grain size of more than 10 μm and less than 20 μm and an average grain size of carbide in the ferrite grain of 0.3 to 1.5 μm It was newly found that the mold life and the moldability (side bend elongation) after FB processing are remarkably improved.

  In FB processing, materials are processed with zero clearance and compressive stress. As a result, the material is subject to significant deformation and cracks may occur during the deformation. When a crack occurs, a fracture surface appears on the punched surface. It is said that spheroidization of carbide is important for preventing cracks. However, when carbides are coarsely present in the ferrite grains, voids are likely to be generated between the carbides during large deformation, and crack formation due to void growth is unavoidable. The sex was investigated. Further, regarding the mold life, the present inventors have inferred that the presence of fine carbides in the ferrite grains promotes the wear of the tool cutting edge and decreases the mold life. Furthermore, the present inventors considered that when the forming process is performed after the FB processing, cracks generated during the FB processing are connected to each other to cause a decrease in forming processability.

First, the experimental results on which the present invention is based will be described.
Hot rolling consisting of high-carbon steel slab (corresponding to S35C) containing 0.34% C-0.2% Si-0.8% Mn in mass%, heated to 1150 ° C, and then rough rolling for 5 passes and finish rolling for 7 passes Thus, a hot rolled steel sheet having a thickness of 4.2 mm was obtained. The total rolling reduction in finish rolling of hot rolling is changed to 10 to 40%, the rolling end temperature is 860 ° C., the winding temperature is 600 ° C., and the cooling rate is air cooled (5 ° C./s) after finish rolling. The temperature was changed to 250 ° C./s and cooled. In addition, the cooling stop temperature when performing cooling other than air cooling (forced cooling) was 650 ° C. Then, after pickling these hot-rolled steel sheets, batch annealing (720 ° C. × 40 h) was performed as hot-rolled sheet annealing.

First, the metal structure of the steel sheet subjected to the hot-rolled sheet annealing was observed.
In the observation of the metal structure, a test piece was taken from the obtained steel plate, the cross section parallel to the rolling direction of the test piece was polished and subjected to nital corrosion. ), The metal structure was observed and imaged, and the ferrite grain size and the ferrite grain size were measured.

  The ferrite grain size and the carbide grain size in the ferrite grain were quantified by image analysis processing using the image analysis software “Image Pro Plus ver. 4.0” manufactured by Media Cybernetics, for the imaged structure. The ferrite grain size was determined by measuring the area of each ferrite grain and determining the equivalent circle diameter from the obtained area. The obtained ferrite grain sizes were arithmetically averaged, and the value was defined as the ferrite average grain size of the steel sheet.

  Further, in the imaged structure, the carbides present on the ferrite grain boundaries are distinguished from the carbides present in the ferrite grains by image analysis. For each carbide present in the ferrite grains, two points on the outer periphery of the carbide and the carbide The diameter passing through the center of gravity of an equivalent ellipse (an ellipse having the same area as the carbide and having the same first and second moments) was measured in increments of 2 ° to obtain the equivalent circle diameter, and each carbide particle size was determined. The obtained carbide particle diameters were arithmetically averaged, and the value was defined as the carbide average particle diameter of the steel sheet. The measured number of carbide grains was 3000.

  In addition, a test piece (size: 100 × 80 mm) was collected from the obtained steel plate, and an FB test was performed. The FB test uses a 110-ton hydraulic press to machine a sample of size: 60 mm x 40 mm (corner radius R: 10 mm) from the test piece, clearance between tools: 0.060 mm (1.5% of the plate thickness), Punched under the conditions of force: 8.5 tons and lubrication. The surface roughness (ten-point average roughness Rz) of the punched sample end face (punched surface) was measured to evaluate the FB workability. In addition, in order to remove the influence of the plate thickness deviation on the clearance, the test piece was ground in equal amounts on both sides in advance to make the plate thickness 4.0 ± 0.010 mm.

The surface roughness is measured on four end faces excluding the R part, and each end face (plate thickness surface) is in the range from 0.5 mm punch side surface to 3.9 mm in the plate thickness direction as shown in FIG. (X direction) 10mm area is scanned 35 times at 100μm pitch in the plate thickness direction (t direction) with a stylus type surface roughness meter, and in accordance with JIS B 0601-1994 The surface roughness Rz was measured. Furthermore, the surface roughness Rz of the measurement surface was obtained by adding the Rz of each scanning line to the average value. Measure the four end faces in the same way as above,
Rz ave = (Rz 1+ Rz 2+ Rz 3+ Rz 4) / 4
(Here, Rz 1, Rz 2, Rz 3, Rz 4: Rz of each surface)
Average surface roughness defined by: R z ave (μm) was calculated.

In general, when the appearance of a fracture surface on the punched end face is 10% or less, it is considered “excellent in FB workability”. However, in the present invention, the smaller the average surface roughness: Rz ave is 10 μm or less, the better the FB workability is. It was excellent.
Moreover, the lifetime of the used tool (mold) was evaluated. The surface roughness (10-point average roughness Rz) of the sample end face (punched surface) when the number of punches in FB processing reached 30000 times was measured in the same manner as described above, and the die life was evaluated.

In addition, a test piece (size: 40 mm x 170 mm (rolling direction)) was punched out from the obtained steel sheet by FB processing, a side bend test was performed, and workability after FB processing (side bend elongation) was evaluated. . FB processing was performed under the conditions of clearance between tools: 0.060 mm (1.5% of the plate thickness), processing force: 8.5 tons, and lubrication: with.
The side bend test was conducted in a state where the side surface (plate surface) of the test piece was constrained in accordance with the method of Nagai et al. (Minai Nagai, Yasutomo Nagai: PK Technical Report, N0.6 (1995), p14). The test was carried out and the elongation at the time of sheet thickness through cracking was measured. The end face of the test piece on the side to be evaluated for elongation was the FB processed surface on the 170 mm length side. The test piece was marked with a mark for evaluating the elongation at break with a distance between the marks of 50 mm. The number of tests was two steel plates, and the average value of the obtained elongation values was defined as the side bend elongation value.

The ferrite average grain size and the average carbide grain size in the ferrite grains varied depending on the total rolling reduction in the hot rolling finish rolling and the average cooling rate after the finish rolling. The obtained results are shown in FIGS.
FIG. 1 shows the relationship between the ferrite average grain size and the side bend elongation. As can be seen from FIG. 1, when the average ferrite grain size exceeds 10 μm, the side bend elongation exceeds 45%, indicating a very good value, and showing good workability after FB processing. When the average ferrite grain size was 20 μm or more, burrs after FB processing became large, and FB processability deteriorated. FIG. 2 shows the relationship between the average carbide grain size in the ferrite grains and the average surface roughness Rz ave of the FB processed punched surface when the average ferrite grain diameter is more than 10 μm and less than 20 μm. From FIG. 2, it can be seen that when the average grain size in the ferrite grains is 1.5 μm or less, Rz ave is 10 μm or less, indicating good FB workability. When the average grain size of ferrite grains in the ferrite grains was less than 0.3 μm, the average surface roughness of the punched surface after punching 30000 times exceeded 10 μm, and the mold life decreased.

The present invention has been completed based on the above findings and further research. That is, the gist of the present invention is as follows.
(1) A composition comprising, by mass%, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.02% or less, the balance being Fe and inevitable impurities And having a structure mainly composed of ferrite and carbide, the average particle diameter of the ferrite is more than 10 μm and less than 20 μm, and among the carbides, the average particle diameter of the carbide present in the ferrite grains is 0.3 to 1.5 μm. A steel plate with excellent fine blanking workability.

(2) In (1), in addition to the said composition, it is set as the composition containing Al: 0.1% or less by the mass% further, The steel plate characterized by the above-mentioned.
(3) In (1) or (2), in addition to the above composition, in addition to mass, Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% and B: A steel sheet comprising a composition containing one or more selected from 0.0005 to 0.005%.

  (4) In a method for manufacturing a steel sheet, in which hot rolling is performed by heating and rolling a steel material to form a hot-rolled sheet, and hot-rolled sheet annealing for annealing the hot-rolled sheet, the steel material is %, C: 0.1 to 0.5%, Si: 0.5% or less, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.02% or less, and a steel material having a composition composed of the balance Fe and inevitable impurities The total rolling reduction in the temperature range of 800 to 950 ° C. in finish rolling is 25% or more, the finish temperature of finish rolling is 800 to 950 ° C., and after the finish rolling is finished, the hot rolling is performed at 50 ° C./s. Cooling at an average cooling rate of 120 ° C./s or less, stopping the cooling at a temperature in the range of 500 to 700 ° C., and winding at 450 to 600 ° C., and annealing the hot-rolled sheet at an annealing temperature of 600 The manufacturing method of the steel plate excellent in the fine blanking workability characterized by setting it as the process set to -720 degreeC.

(5) In (4), in addition to the said composition, it is set as the composition which contains further Al: 0.1% or less by the mass%, The manufacturing method of the steel plate characterized by the above-mentioned.
(6) In (4) or (5), in addition to the above-mentioned composition, in addition to mass, Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% and B: The manufacturing method of the steel plate characterized by setting it as the composition containing 1 type, or 2 or more types chosen from 0.0005-0.005%.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate which was excellent in FB workability, and was excellent also in the workability (side bend elongation) after FB processing can be manufactured easily and cheaply, and there exists a remarkable industrial effect. In addition, according to the present invention, the steel sheet has excellent FB workability, it is not necessary to perform end face processing after FB processing, the manufacturing period can be shortened, the productivity can be improved, and the manufacturing cost can be reduced. There is also an effect of becoming.

First, the reasons for limiting the composition of the steel sheet of the present invention will be described. The mass% in the composition is simply expressed as% unless otherwise specified.
C: 0.1-0.5%
C is an element that affects the hardness after hot rolling annealing and after quenching, and in the present invention, it is necessary to contain 0.1% or more. If C is less than 0.1%, the hardness required for automobile parts cannot be obtained. On the other hand, if the content exceeds 0.5%, the steel sheet becomes hard, so that an industrially sufficient mold life cannot be secured. For this reason, C was limited to the range of 0.1 to 0.5%.

Si: 0.5% or less
Si is an element that acts as a deoxidizer and increases the strength (hardness) by solid solution strengthening, but if it is contained in a large amount exceeding 0.5%, the ferrite becomes hard and the FB workability is lowered. If Si is contained in excess of 0.5%, a surface defect called red scale occurs at the hot rolling stage. For this reason, Si was limited to 0.5% or less. In addition, Preferably it is 0.35% or less.

Mn: 0.2-1.5%
Mn is an element that effectively increases the strength of the steel by solid solution strengthening and effectively improves the hardenability. In order to obtain such an effect, it is desirable to contain 0.2% or more. However, if it exceeds 1.5%, the solid solution strengthening becomes too strong, the ferrite becomes hard, and the FB workability decreases. For this reason, Mn was limited to the range of 0.2 to 1.5%. In addition, Preferably, it is 0.2 to 1.0%, More preferably, it is 0.6 to 0.9%.

P: 0.03% or less P is segregated at grain boundaries and reduces workability. Therefore, in the present invention, it is desirable to reduce it as much as possible, but 0.03% is acceptable. For these reasons, P is limited to 0.03% or less. In addition, Preferably it is 0.02% or less.
S: 0.02% or less S is an element that forms sulfides such as MnS in steels as inclusions and lowers FB workability. It is desirable to reduce it as much as possible, but it is acceptable up to 0.02%. . For these reasons, S is limited to 0.02% or less. In addition, Preferably it is 0.01% or less.

In the present invention, in addition to the above basic composition, one or more selected from Al, and / or Cr, Mo, Ni, Ti and B is contained in the present invention. it can.
Al: 0.1% or less
Al is an element that acts as a deoxidizer and combines with N to form AlN, thereby contributing to prevention of austenite grain coarsening. When it is contained together with B, it also has an effect of fixing N and preventing a reduction in the amount of B effective for improving hardenability. Such an effect becomes remarkable when the content is 0.02% or more, but the content exceeding 0.1% lowers the cleanliness of the steel. For this reason, when it contains, it is preferable to limit Al to 0.1% or less. In addition, Al as an inevitable impurity is 0.01% or less.

Cr, Mo, Ni, Ti, and B are all elements that contribute to improving hardenability or further improving temper softening resistance, and can be selected and contained as necessary.
Cr: 3.5% or less
Cr is an element effective for improving hardenability. To obtain such an effect, it is preferable to contain 0.1% or more. However, if it exceeds 3.5%, FB workability deteriorates and temper softening occurs. This causes an excessive increase in resistance. For this reason, when Cr is contained, it is preferably limited to 3.5% or less. More preferably, it is 0.2 to 1.5%.

Mo: 0.7% or less
Mo is an element that effectively works to improve hardenability. In order to obtain such an effect, it is preferable to contain 0.05% or more. However, if it exceeds 0.7%, the steel is hardened and FB processing is performed. Sexuality decreases. For this reason, when it contains Mo, it is preferable to limit to 0.7% or less. More preferably, it is 0.1 to 0.3%.

Ni: 3.5% or less,
Ni is an element that improves hardenability. In order to obtain such an effect, Ni is preferably contained in an amount of 0.1% or more. However, if it exceeds 3.5%, the steel becomes hard and the FB workability decreases. . For this reason, when it contains Ni, it is preferable to limit to 3.5% or less. In addition, More preferably, it is 0.1 to 2.0%.

Ti: 0.01-0.1%
Ti is an element that easily binds to N to form TiN and effectively acts to prevent coarsening of γ grains during quenching. Further, when it is contained together with B, N forming BN is reduced, so that the addition amount of B necessary for improving the hardenability can be reduced. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, if the content exceeds 0.1%, ferrite is precipitated and strengthened by precipitation of TiC or the like, resulting in a hardened mold life. For this reason, when it contains, it is preferable to limit Ti to the range of 0.01 to 0.1%. In addition, More preferably, it is 0.015 to 0.08%.

B: 0.0005-0.005%
B is an element that segregates at the austenite grain boundaries and improves the hardenability in a small amount, and is particularly effective when combined with Ti. In order to improve hardenability, a content of 0.0005% or more is required. On the other hand, even if the content exceeds 0.005%, the effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, when it contains, it is preferable to limit B to 0.0005 to 0.005% of range. In addition, More preferably, it is 0.0008 to 0.004%.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include N: 0.01% or less, O: 0.01% or less, and Cu: 0.1% or less, for example.
Next, the reason for limiting the structure of the steel sheet of the present invention will be described.
The steel sheet of the present invention has a structure mainly composed of ferrite and carbide. The structure mainly composed of ferrite and carbide is a structure in which the volume ratio of ferrite and carbide is 95% or more. In other words, the steel sheet of the present invention has a composition consisting essentially of ferrite and carbide, but can tolerate phases other than ferrite and carbide up to about 5% by volume.

  In the steel sheet of the present invention, the average particle size of ferrite is more than 10 μm and less than 20 μm. When the average grain size of ferrite is 10 μm or less, as shown in FIG. 1, the side bend elongation after FB processing decreases. The reason for this is not clear, but the present inventors have inferred that when the average ferrite grain size is reduced to 10 μm or less, the diffusion rate is fast at the ferrite grain boundary, and the average grain size of carbides present at the ferrite grain boundary increases. Therefore, voids are likely to occur between the carbides on the ferrite grain boundaries due to large deformation during FB processing, and cracks are likely to occur.The cracks propagate and coalesce during molding after FB processing. It is thought that the side bend elongation decreased. On the other hand, if the ferrite average particle size is 20 μm or more, the burr height after processing is remarkably increased, although the softening is achieved and the mold life is improved. For this reason, the average grain size of ferrite is limited to more than 10 μm and less than 20 μm. In addition, Preferably it is 12-18 micrometers.

  Moreover, in this invention steel plate, the average particle diameter of the carbide | carbonized_material in a ferrite grain is made into the range of 0.3-1.5 micrometers among carbide | carbonized_materials. If the average particle size of the carbides present in the ferrite grains is less than 0.3 μm, the steel plate becomes hard and the mold life is shortened. On the other hand, when the grain size is increased beyond 1.5 μm, voids are generated between the carbides due to large deformation during FB processing, as shown in Fig. 2, grows into cracks, generates a fracture surface, and roughs the machined surface (punched surface). Increases FB workability. For this reason, the average particle size of the carbides in the ferrite grains is limited to a range of 0.3 to 1.5 μm.

Below, the preferable manufacturing method of this invention steel plate is demonstrated.
It is preferable to melt the molten steel having the above-described composition by a conventional melting method such as a converter and to obtain a steel material (slab) by a conventional casting method such as a continuous casting method.
Subsequently, the obtained steel material is hot-rolled by heating and rolling the steel material to form a hot-rolled sheet.
In hot rolling, the total rolling reduction in the temperature range of 800 to 950 ° C. in finish rolling is 25% or more, the finish temperature of finish rolling is 800 to 950 ° C., and after the finish rolling is finished, 50 ° C./s or more and 120 ° C. Cooling is performed at an average cooling rate of less than / s, the cooling is stopped at a temperature in the range of 500 to 700 ° C., and winding is performed at 450 to 600 ° C.

  In the hot rolling in the present invention, a hot rolled steel sheet having a pearlite structure of almost 100% can be obtained by adjusting the finishing temperature of finish rolling and the subsequent cooling conditions. Furthermore, in the hot rolling in the present invention, the average grain size of ferrite exceeds 10 μm after proper hot-rolled sheet annealing by setting the total rolling reduction in the temperature range of 800 to 950 ° C. in finish rolling to 25% or more. A tissue that is less than 20 μm is obtained.

Total rolling reduction in the temperature range of 800 to 950 ° C in finish rolling: 25% or more In finishing rolling in hot rolling, the austenite grain size is reduced by increasing the rolling reduction, and accordingly the pearlite grain size after transformation In the hot-rolled sheet annealing, the growth of ferrite grains is promoted by using the high grain boundary energy of fine pearlite as a driving force.

Here, especially at a high temperature exceeding 950 ° C., the austenite grain size tends to increase due to recrystallization, and therefore the influence of the reduction in the temperature range of 950 ° C. or less is large.
Pearlite changes into polygonal ferrite and spherical cementite by hot-rolled sheet annealing. In order to make the average grain size of the ferrite produced by this hot-rolled sheet annealing more than 10 μm and less than 20 μm, the total rolling reduction in the temperature range of 800 to 950 ° C. in the finish rolling is a rolling reduction larger than that in the usual rolling. 25% or more. When the total rolling reduction in the temperature range of 800 to 950 ° C. is less than 25%, the rolling reduction is insufficient, and it becomes difficult to make the ferrite grain size within a desired range. The upper limit of the total rolling reduction is preferably 35% or less from the viewpoint of rolling load. In addition, More preferably, it is 25 to 33%.

Finishing rolling finish temperature: 800 ~ 950 ℃
If the finishing temperature of finish rolling exceeds 950 ° C., the generated scale becomes thick and the pickling property decreases, and a decarburized layer may be formed on the steel sheet surface layer, and the ferrite grain size tends to be coarse. On the other hand, when the finishing temperature of finish rolling is less than 800 ° C., the increase in rolling load becomes significant, and an excessive load on the rolling mill becomes a problem. For this reason, it is preferable to make the finish temperature of finish rolling into the temperature within the range of 800-950 degreeC.

Average cooling rate after finishing rolling: 50 ° C / s or more and less than 120 ° C / s After finishing rolling, cooling is performed at an average cooling rate of 50 ° C / s or more. The average cooling rate is an average cooling rate from the finish rolling finish temperature to the cooling (forced cooling) stop temperature. When the average cooling rate is less than 50 ° C / s, ferrite that does not contain carbides is generated during cooling, and the structure after cooling becomes a non-uniform structure of ferrite + pearlite, ensuring a uniform structure consisting of almost 100% pearlite. Disappear. If the hot rolled sheet structure is non-uniform ferrite + pearlite, the distribution of carbides will be non-uniform, and no matter how the subsequent hot rolled sheet annealing is done, the carbides present in the grains tend to be coarse, so finish rolling. It is preferable to limit the average cooling rate after completion to 50 ° C./s or more. The average cooling rate after finishing rolling is preferably less than 120 ° C./s from the viewpoint of preventing the formation of bainite. When the average cooling rate is 120 ° C / s or more, the structure tends to be different between the steel sheet surface layer part and the sheet thickness center part, and after hot-rolled sheet annealing, the deformability is different between the surface layer part and the sheet thickness center part. FB processability and molding processability after FB processing are likely to deteriorate. For this reason, the average cooling rate after finishing rolling is preferably set to 50 ° C./s or more and less than 120 ° C./s.

Cooling stop temperature: 500-700 ° C
The temperature at which the cooling (forced cooling) is stopped is preferably 500 to 700 ° C. When the cooling stop temperature is less than 500 ° C., problems such as hard bainite and martensite are generated and hot-rolled sheet annealing takes a long time, and operational problems such as cracking during winding occur. On the other hand, when the cooling stop temperature exceeds 700 ° C, the ferrite transformation nose is around 700 ° C, so ferrite is generated during cooling after the cooling stop, ensuring a uniform structure consisting of almost 100% pearlite. become unable. For this reason, the cooling stop temperature is preferably limited to a temperature within the range of 500 to 700 ° C. In addition, More preferably, it is 500-650 degreeC, More preferably, it is 500-600 degreeC.

After the cooling is stopped, the hot rolled plate is immediately wound into a coil.
Winding temperature: 450-600 ° C
When the coiling temperature is less than 450 ° C., cracks occur in the steel sheet during coiling, which causes operational problems. On the other hand, when the winding temperature exceeds 600 ° C., there is a problem that ferrite is generated during winding. In addition, Preferably it is 500-600 degreeC.
The hot-rolled steel sheet (hot-rolled steel sheet) thus obtained is then subjected to hot-rolled sheet annealing at an annealing temperature of 600 to 720 ° C. after the surface oxide scale is removed by pickling or shot blasting. Is done. By subjecting a hot-rolled sheet having a pearlite structure of almost 100% to appropriate hot-rolled sheet annealing, the spheroidization of the carbide is promoted and the ferrite grain size is adjusted to a desired range, and the carbide grains in the ferrite grain The diameter can be adjusted to a predetermined range.

Annealing temperature of hot-rolled sheet annealing: 600 ~ 720 ℃
When the annealing temperature is less than 600 ° C., the average particle size of the carbide in the ferrite grains is less than 0.3 μm. On the other hand, when the temperature is higher than 720 ° C., the average particle size of the carbide in the ferrite grains exceeds 1.5 μm, and the FB workability is lowered. The holding time for hot-rolled sheet annealing is not particularly limited, but is preferably 8 hours or longer in order to adjust the carbide particle size within a desired range. Further, if it exceeds 80 h, the ferrite grains become excessively coarse, and the average grain size of the ferrite grains in the ferrite grains may exceed 1.5 μm.

  A steel material (slab) having the composition shown in Table 1 was used as a starting material. These steel materials were heated to the heating temperatures shown in Table 2, and then hot rolled sheets having a thickness of 4.2 mm were formed under the hot rolling conditions shown in Table 2. As hot rolling conditions, the total rolling reduction in the temperature range of 800 ° C. to 950 ° C. in finish rolling, the rolling end temperature of finish rolling, the average cooling rate in cooling after finishing rolling, the cooling stop temperature, and the winding temperature are changed. It was.

These hot-rolled sheets were then subjected to batch annealing and pickling treatment. The obtained steel sheet was evaluated for structure observation, FB workability, and workability after FB processing (side bend elongation). The test method is as follows.
(1) Structure observation A specimen for structure observation was collected from the obtained steel sheet. Then, after polishing the cross section parallel to the rolling direction of the test piece and corroding the nital, metal with a scanning electron microscope (SEM) (magnification, ferrite: 1000 times, carbide: 3000 times) at the 1/4 thickness position Observe the structure (number of fields of view: 30 places) and use image analysis software “Image Pro Plus ver.4.0” manufactured by Media Cybernetics to analyze the volume fraction of ferrite and carbide, ferrite grain size, ferrite Intragranular carbide particle size was measured.

The volume ratio of ferrite and carbide was determined by observing the metal structure with SEM (magnification: 3000 times) (number of fields: 30), and adding the area of ferrite and carbide area excluding carbide to the total field of view. The area ratio was calculated by dividing the volume ratio of ferrite and carbide.
The ferrite grain size was determined by measuring the area of each ferrite grain and determining the equivalent circle diameter from the obtained area. The obtained ferrite grain sizes were arithmetically averaged, and the value was defined as the ferrite average grain size of the steel sheet. The measured area ratio was 500 pieces each.

  The grain size of the carbide in the ferrite grain is identified in each field of view (number of fields: 30) in the metal structure observation (magnification: 3000 times), and the carbide present in the ferrite grain is identified by image analysis and present in the ferrite grain. For each carbide to be measured, measure the diameter passing through the center of gravity of the two points on the outer circumference of the carbide and the equivalent ellipse of the carbide (the ellipse with the same area and the same primary and secondary moment as the carbide) in 2 ° increments to obtain the equivalent circle diameter These were used as the respective carbide particle diameters, and the average value of the obtained carbide particle diameters was taken as the average particle diameter of the ferrite grains. The measured number of carbide grains was 3000.

(2) FB workability A test piece (size: 100 × 80 mm) was collected from the obtained steel sheet and subjected to FB test. The FB test uses a 110-ton hydraulic press to machine a sample of size: 60 mm x 40 mm (corner radius R: 10 mm) from the test piece, clearance between tools: 0.060 mm (1.5% of the plate thickness), Punched under the conditions of force: 8.5 tons and lubrication. The surface roughness (ten-point average roughness Rz) of the punched sample end face (punched surface) was measured to evaluate the FB workability. In addition, in order to remove the influence of the plate thickness deviation on the clearance, the test piece was ground in equal amounts on both sides in advance to make the plate thickness 4.0 ± 0.010 mm.

The surface roughness is measured at four end faces excluding the R part, and each end face (thickness face) is in the range from 0.5mm punch side surface to 3.9mm in the thickness direction as shown in Fig. 3. (X direction) 10mm area was scanned 35 times at 100μm pitch in the plate thickness direction (t direction) with a stylus type surface roughness meter, and in each scanning line according to JIS B 0601-1994 The surface roughness Rz was measured. Furthermore, the surface roughness Rz of the measurement surface was obtained by adding the Rz of each scanning line to the average value. Measure the four end faces in the same way as above,
Rz ave = (Rz 1+ Rz 2+ Rz 3+ Rz 4) / 4
(Here, Rz 1, Rz 2, Rz 3, Rz 4: Rz of each surface)
The average surface roughness defined by the formula: R z ave (μm) was calculated, and the FB workability was evaluated.

As described above, in the present invention, the smaller the Rz ave is 10 μm or less, the better the FB workability.
In addition, we observed the occurrence of large burrs (high burrs) that would cause problems in FB processing.
Moreover, the lifetime of the used tool (mold) was evaluated. The surface roughness (10-point average roughness Rz) of the sample end face (punched surface) when the number of punches in FB processing reached 30000 times was measured, and the mold life was evaluated. The method for measuring the surface roughness was the same as that described above. The average surface roughness Rz ave of the sample end face was evaluated as ◯ when 10 μm or less, Δ when exceeding 10 μm to 16 μm or less, and × when exceeding 16 μm.

(3) Workability after FB processing (side bend elongation)
A test piece (size: 40 mm × 170 mm (rolling direction)) was punched from the obtained steel sheet by FB processing, a side bend test was performed, and workability after FB processing (side bend elongation) was evaluated. In addition, in order to remove the influence of the thickness deviation with respect to the clearance, the test piece was ground in advance by equal amounts on both sides to obtain a thickness of 4.0 ± 0.10 mm. FB processing was performed under the conditions of clearance between tools: 0.060 mm (1.5% of the plate thickness), processing force: 8.5 tons, and lubrication: with.

  The side bend test was conducted in a state where the side surface (plate surface) of the test piece was constrained in accordance with the method of Nagai et al. (Minai Nagai, Yasutomo Nagai: PK Technical Report, N0.6 (1995), p14). The test was carried out and the elongation at the time of sheet thickness through cracking was measured. The end face of the test piece on the side to be evaluated for elongation was the FB processed surface on the 170 mm length side. The test piece was marked with a mark for evaluating the elongation at break with a distance between the marks of 50 mm. The number of tests was two for each steel plate, and the average value of the obtained elongation values was taken as the side bend elongation value. When the side bend elongation value was 45% or more, ○ was evaluated, and when the side bend elongation value was less than 45%, x was evaluated for workability after FB processing (side bend elongation).

  The obtained results are shown in Table 3.

  In all of the examples of the present invention, the average surface roughness Rz ave of the punched surface is 10 μm or less, excellent in FB workability, and the surface of the punched surface at the time of punching: 30000 times is smooth (evaluation: ◯) There is no reduction in mold life. Moreover, the example of this invention is excellent also in the side bend elongation (workability) after FB processing. In the examples of the present invention, the total volume ratio of ferrite and carbide was 95% or more, and it was confirmed that the structure was mainly composed of ferrite and carbide. On the other hand, the comparative example out of the scope of the present invention is that the average surface roughness Rz ave of the punched surface becomes rougher than 10 μm and the FB workability deteriorates, or a large burr occurs during FB processing, or the mold The service life is reduced, or the side bend elongation (workability) after FB processing is reduced, or the FB workability, mold life, and side bend elongation (workability) after FB processing are all reduced. is doing.

3 is a graph showing the relationship between the average ferrite grain size and the side bend elongation after FB processing. It is a graph which shows the relationship between FB workability (average surface roughness of a punching surface: Rz ave) and the average grain size of carbide in ferrite grains. It is explanatory drawing which illustrates typically the surface roughness measurement area | region of the punching surface after FB process.

Claims (6)

% By mass
C: 0.1 to 0.5%, Si: 0.5% or less,
Mn: 0.2 to 1.5%, P: 0.03% or less,
S: containing 0.02% or less, balance Fe and inevitable impurities, and a structure mainly composed of ferrite and carbide, and the ferrite has an average particle size of more than 10 μm and less than 20 μm. A steel plate excellent in fine blanking workability, characterized in that the average particle size of carbides present therein is 0.3 to 1.5 μm.   In addition to the said composition, it is set as the composition containing Al: 0.1% or less by the mass% further, The steel plate of Claim 1 characterized by the above-mentioned.   In addition to the above-mentioned composition, one kind selected from Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% and B: 0.0005 to 0.005% by mass% Or it is set as the composition containing 2 or more types, The steel plate of Claim 1 or 2 characterized by the above-mentioned. In the method of manufacturing a steel sheet, in which hot rolling is performed by heating and rolling a steel material to form a hot rolled sheet, and hot rolled sheet annealing for annealing the hot rolled sheet,
The steel material in mass%,
C: 0.1 to 0.5%, Si: 0.5% or less,
Mn: 0.2 to 1.5%, P: 0.03% or less,
S: A steel material having a composition including 0.02% or less and the balance Fe and inevitable impurities,
In the hot rolling, the total rolling reduction in the temperature range of 800 to 950 ° C. in finish rolling is 25% or more, the finish temperature of finish rolling is 800 to 950 ° C., and after the finish rolling is finished, 50 ° C./s or more and 120 Cooling is performed at an average cooling rate of less than ℃ / s, and the cooling is stopped at a temperature in the range of 500 to 700 ℃, and winding is performed at 450 to 600 ℃, and the hot-rolled sheet annealing is performed at an annealing temperature of 600 to 720. A method for producing a steel sheet excellent in fine blanking workability, characterized in that the treatment is performed at ℃.   The method for producing a steel sheet according to claim 4, wherein in addition to the composition, the composition further contains, by mass%, Al: 0.1% or less.   In addition to the above-mentioned composition, one kind selected from Cr: 3.5% or less, Mo: 0.7% or less, Ni: 3.5% or less, Ti: 0.01 to 0.1% and B: 0.0005 to 0.005% by mass% Or it is set as the composition containing 2 or more types, The manufacturing method of the steel plate of Claim 4 or 5 characterized by the above-mentioned.

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